(Not applicable)
This invention relates to the manufacture of golf club shafts and other shafts and poles from fiber/resin composite material.
A common method for forming golf shafts from fiber/resin composite materials is to wrap fiber impregnated fiber around hard tooling in the form of a mandrel which defines the interior dimensions of the shaft. The fiber is compacted during the winding, by using, for example, rolling tables, and pressure may also be applied by a shrinkable tape wrapped around the mandrel. The advantage of these systems is that the shape and dimensions of uncured resin/fiber preform as it is wound or formed on the mandrel closely approximate the final product. There is little movement of the fiber during cure of the composite, thus the positions of the reinforcing fibers can well be controlled. However, these methods have the disadvantage that only low compaction pressures can only be applied if oven cure of the composite is used. Pressures can be applied using autoclave equipment, but this is not economically feasible for most applications. High compaction pressures are required to form high performance composites that are of light weight but have high strength. In addition, since the tooling must be removable after curing of the part, complex profiles which would trap the rigid interior mandrel pieces cannot be made with this method. Accordingly, this method is limits the shafts to lower structural performance, since the composite is made with low compaction pressure, and this method is also limited to relatively simple tapered shapes.
Another approach is disclosed in French patent applications 90 15387 and 90 15388. In this method a bladder is formed upon a hard metal mandrel. Resin impregnated fiber is formed around the bladder and the assembly placed into female tooling. The bladder is inflated at the base of the mandrel to expand the impregnated fiber into the tooling. This method has several disadvantages. The method disclosed is limited to shapes where the rigid mandrel can be removed from the cured part, i.e., is not trapped by the profile of shape of the part. Conversely, the impregnated fiber material in the preform would have to move substantially from its initial as formed or rolled shape to form a complex shape, which would likely displace the fibers and possibly compromise the properties of the final part. In addition, creation of a complex shape with, for example, a bulge would have entrances precluding removal of a rigid mandrel. Furthermore, the bladder used in this process is a thin walled latex rubber material formed by dipping the mandrel. Accordingly, the bladder necessarily is thin walled and has essentially the shape of the mandrel. In addition, the bladder such a bladder would not be durable and be capable of repeated use. The process of the French patent application also necessarily leaves the rigid metal mandrel inside the part during cure. This has the disadvantage of substantially slowing the rates of heat up and cool down during cure, because the large thermal mass of the metallic mandrel is thermally insulated by the relatively low conductivity composite material. Also, a large number of rigid mandrels are needed to flow through the entire bladder fabrication, shaft fabrication process, substantially increasing capital equipment costs.
The present process involves an improvement of the process disclosed in U.S. Pat. No. 5,534,203, issued Jul. 9, 1996, which is hereby incorporated by reference. In this method plastic laminates are laid up around an inflatable bladder and placed in a female mold. The bladder inflates to compact the laminates to substantially eliminate voids and form a smooth, hollow shaft with a structurally strong and stiff skin. The outer dimensions of the shaft are defined by the dimensions of the female mold. This manufacturing process can generally be referred to as a xe2x80x9cmolding processxe2x80x9d or the shafts can be referred to as xe2x80x9cmolded shaftsxe2x80x9d. A particular advantage of this system is that since there is no internal mandrel that must be removed, complex shapes can be formed. In addition, the compression from the bladder is significantly higher that in fixed mandrel processes, which significantly improves the properties of the final part. However, a problem with this process, particular in the formation of golf shafts, is that the movement of the impregnated fiber from inflation of the bladder can displace the fiber sufficiently to compromise the properties of the cured part.
In the present process, the advantages of the molding process are retained while minimizing the movement of impregnated fiber during inflation of the bladder. Thus, the advantages of the molding process are combined with the advantages of fixed internal mandrel processes.
The advantages of the present invention are provided by providing an inflatable bladder that functions as a mandrel in that it is used to form a preform with the same approximate dimensions of the final part. The bladder is dimensioned so that any expansion of the bladder and the preform is minimized when the bladder is pressurized to compress the preform against the female tooling. The preform must only be slightly smaller than the inner dimensions of the female tooling to permit insertion of the bladder and preform into the tooling.
By forming the preform around a bladder that closely defines the shape of the final part, the advantages of the fixed mandrel process is achieved, since there is little movement of the fiber during the subsequent curing process. Since, there is no hard mandrel that has to be removed, but a flexible bladder, complex profiles of the shaft can be achieved and the shafts of the invention are not limited to simple tapers.
The invention can be viewed as the formation of a preform around a pressurizable rubber mandrel, the mandrel providing both the way to closely defined the shape of the preform to the final shape as well as provide the way for compressing the preform against female tooling that defines the outer dimensions of the part. Thus, the bladder, functioning as a mandrel, provides the advantage avoiding material fiber displacement by defining the preform shape to closely correspond to the final shape, but since the bladder is of a flexible material it can be removed from shafts having complex profiles. The bladder also has the function of providing a compressive force to compress the preform during curing, by pressing the preform against female tooling. Since the preform need only be slightly smaller to enable placement of the preform in the female tooling, only minimum expansion of the preform results from the pressurization, with little or no displacement of fiber. In preferred practice, a rigidizer, in the form of metal mandrel is inserted in the shaped elastomeric bladder of nonuniform wall thickness, to provide a more or less rigid substrate for preform construction. The rigid interior mandrel has conical shape with gradual, continuous, and perhaps varying rates of taper down its length, which allow it to be easily extracted after preformed construction and before cure.
The invention is basically a dual function molding tool, a shaped bladder of nonuniform wall thickness which (1) serves as a dimensional template like a mandrel in the formation of the preform, and (2) a systems for compressing the preform to provide for a high-performance composite material. Furthermore, the shaped elastomeric bladder with non-uniform wall thickness can be easily removed from interior spaces which would trap a rigid mandrel. The removal of the shaped elastomeric bladder from a trapped shape is achieved by pulling on, and thereby stretching the bladder to a smaller diameter. In contrast, rigid tooling that would be removable would be of a segmented multipiece design, which would not be economical or practical.
The invention allows for manufacture of shafts of varying diameter where, (1) the preform is dimensioned closely to final dimensions of the shaft, and (2) the shaft may have a xe2x80x9ctrapped shape.xe2x80x9d A trapped shape is a waste or bulge along the length of the shaft that would render removal of a hard metal interior mandrel impossible, e.g. where a middle portion of the shaft has a diameter larger or smaller than adjacent portions above and below. Traditional hard interior hard mandrel methods cannot be used to form trapped-shape shafts. Molding methods with interior bladders can be used for trapped shapes but these methods cannot form shafts where the preform is closely dimensioned to the final the final shape. Accordingly, there must be significant movement of the preform when the bladder is inflated. The present invention is able to combine the advantages of both systems by providing a pressurized bladder that functions as a mandrel to shape the preform, but is flexible enough to be removed from trapped-shaped shafts.
In practice, the golf shafts of the invention have a will thickness of about 0.02 inches to about 0.09. The outer dimensions of the bladder are only slightly smaller than the inner dimensions of golf shaft, about ⅓ of the wall thickness of the shaft or about 10 to 20 thousandths of the an inch.
The shafts of the invention are made from conventional impregnated fibers, such as carbon, graphite, fiberglass, KEVLAR(trademark) polyamide, and boron fibers. The fibers are impregnated with any of various thermoplastic and thermosetting resins, and can be formed into hardened fiber reinforced shapes. Preferably, the shafts of the invention are made from carbon or graphite impregnated with epoxy resins. Fiber preimpregnated with resin, i.e., xe2x80x9cprepreg,xe2x80x9d is commercially available, or fibers may be impregnated with suitable resins.
The process of the invention is particularly applicable to the manufacture of golf shafts that are circular in cross-section and are essentially radially uniform with respect to flexural properties. The shafts can be of complex shape, however, along the longitudinal axis, and is adapted to for making composite shafts that are narrow waisted, or have bulges along its length, as the so-called xe2x80x9cbubble-shafts,xe2x80x9d as disclosed, for example, in U.S. Pat. No. 5,692,675, issued Dec. 2, 1997, which is hereby incorporated herein by reference. The process of the invention is also applicable to the manufacture of composite shafts or poles that have a straight longitudinal axis. The method of the invention is particularly advantageous in forming high-performance composite shafts made with high compaction pressures and high fiber volumes, and shafts that have a variable profile along their length.
Referring to FIG. 1, which a generalized flow sheet of the process of the invention, the bladder is preformed, preferably by molding, with dimensions approximating the final composite part, as described elsewhere. The bladder must also be formed with an internal cavity so that it can be pressurized. Any suitable method for manufacture may be used. A preferred method is to mold the bladder around a tapered mandrel that is removed after molding to form a generally tubular form, and sealing one end to form an pressurizable bladder.
The bladder is preferably of an elastomeric, heat-resistant material, such as silicone rubber. The bladder is required to expand but very little when the bladder is pressurized to compress the impregnated fiber against the female tooling, but it must also be stretched and removed from the interior of the finished cured part. Preferably, the bladder is heat-resistant and durable to allow to be used for the manufacture of several composite shafts.
Bladder pressures between 200 psi to 250 psi are commonly used, with pressures up to 350 psi also easily used.
Preimpregnated fiber is formed around the bladder to form a preform closely dimensioned to the final shaft dimensions. Accordingly, the preform is tightly wound, as much as practically possible. Since the preform must be capable of being placed in a female mold, it is made slightly smaller than the final shaft, and having the preform tightly made will decreased the amount that it will have to be displaced by pressurization of the bladder. To assist in applying or rolling the preimpregnated fiber around the bladder, a rolling mandrel is preferably inserted into the cavity of the mandrel to stiffen or rigidize the bladder. The rigid internal rolling mandrel has essentially the same dimensions as the mandrel used to form the inner surface in the bladder during injection molding of the bladder. It is slightly smaller (about 2.5%) to compensate for shrinkage of the elastomeric bladder material as it cures. Correspondingly, the entire bladder-molding tool, and center mandrel are oversized, about 2.5% relative to the desired final dimensions of the bladder.
After the preform is formed around the bladder, the rolling mandrel is removed, and the assembly of the bladder and preform is placed into a female mold. The preform is dimensioned slightly smaller to enable placement in the mold and closure of the mold without pinching of fiber between the mold parts, i.e., to prevent the formation of fiber flash. The amount in which the preform is dimensioned smaller than the mold is preferably as small as practical.
The mold is then closed, and the bladder is pressurized to compress the impregnated fiber against the interior surface of the mold. A vacuum will also typically be applied after the mold is closed and before the bladder is pressurized. The mold is heated to cure or hardened the impregnating resin and to form a hard fiber-reinforced composite. After curing the mold is opened and the finished shaft is removed. Any flash from leakage of the resin between the mold parts is then scraped off. The pressurization and cure times are chosen depending upon the resin/fiber used and the properties required for the part.